1. Field of the Invention
This invention relates to an ion sensor and more particularly to an ion sensitive field effect transistor for measuring a given ion concentration in an electrolyte.
2. Description of the Prior Art
In such kind of ion sensitive field effect transistor, an insulating gate type field effect transistor comprises a chemically selective film containing an ion exchange substance, enzyme or the like and formed on a gate portion thereof, the chemically selective film being operative to detect the presence or the like of a given substance acting on a given active amount of ion or enzyme in an electrolyte.
In a first example of an ion sensitive field effect transistor which has heretofore been proposed, a chemically selective film is formed on that portion of an insulating layer which is superimposed on a channel region and that portion of the insulating layer which is not covered with the chemically selective film is covered with a solution impermeable film. When such conventional ion sensitive field effect transistor is used, the chemically selective film is brought into contact with a test liquid. In this case, the insulating layer covering the surface of a semiconductor substrate is hydrated through the chemically selective film and a given cation, for example, H.sup.+, Na.sup.+ or the like is penetrated into the insulating layer, thereby inducing a gate leak failure or the like and hence inducing a formidable obstacle in the case of measuring stability and reproductive property of the ion sensitive field effect transistor. In addition, the solution impermeable film covering that portion of the insulating layer which is not covered with the chemically selective film and formed of epoxy resin becomes swollen after the lapse of time, thereby exerting bad influence to the stability of the ion sensitive field effect transistor.
In a second example of an ion sensitive field effect transistor which has heretofore been proposed, a silicon nitride layer and an ion sensitive layer are suerpimposed one upon the other on that portion of an insulating layer which is opposed to a channel region to form a gate portion and the outer surface other than this gate portion is covered with an insulating resin layer. In such conventional ion sensitive field effect transistor, it is difficult to make the insulating resin layer covering the surface other than the gate portion thin without forming any pin hole therein. In addition, the insulating resin layer is gradually swollen in a test liquid, thereby degrading the stability and durability of the ion sensitive field effect transistor.
In a third example of an ion sensitive field effect transistor which has heretofore been proposed, a gate portion as a whole is covered with an oxide film and a surface stabilizing film superimposed one upon the other and on that side of the surface stabilizing film which is opposed to drain and source diffusion regions is formed an ion sensitive film.
In all of the above mentioned conventional ion sensitive field effect transistors, the chemically selective film of the first example, ion sensitive layer of the second example and ion sensitive film of the third example are superimposed on the insulating layer, silicon nitride layer and surface stabilizing film by chemical vapor deposition, dipping or the like, respectively. As a result, the ion sensitive film, for example, is not sufficiently adhered to the surface stabilizing film and hence becomes easily separated or effluent from the latter, thereby exposing the surface stabilizing film and hence losing the ability of the ion sensitive field effect transistor within a short time.